1. Field of the Disclosure
The present disclosure relates to an improvement in availability in a passive optical network (PON) in which wavelength multiplexing and time division multiplexing are combined.
2. Discussion of the Background Art
With the recent rapid spread of the Internet, access service systems have been required to be increased in capacity, advanced, and economized, and a passive optical network (PON) has been studied as a means for implementing them. The PON is an optical communication system in which economization is pursued by sharing one optical line terminal and a part of a transmission path by a plurality of optical network units using an optical multiplexer/demultiplexer based on an optical passive element.
Currently, in Japan, an economic optical subscriber system, Gigabit Ethernet (a registered trademark) passive optical network (GE-PON) in which a circuit capacity of 1 Gbps is shared by up to 32 users with time division multiplexing (TDM) has mainly been introduced. This made it possible to provide a fiber to the home (FTTH) service at a realistic rate.
In order to deal with the need of a larger capacity, a 10 Gigabit Ethernet (a registered trademark) passive optical network (10 GE-PON) with a total band of 10 Gbps class has been studied as a next generation optical subscriber system and approved as an international standard in 2009. This is an optical subscriber system in which a large capacity is implemented while using the same one as that of the GE-PON as a transmission path portion such as an optical fiber by increasing a bit rate of a transceiver.
In the future, a large-capacity service exceeding 10 G class such as an ultra high-definition video service or a ubiquitous service is considered to be required, but when the bit rate of the transceiver is simply increased from a 10 G class to a 40/100 G class, there is a problem in that a system upgrade cost is increased, and thus it is difficult to put it to practical use.
As a means for solving this, a wavelength tunable WDM/TDM-PON in which wavelength tenability is added to a transceiver so that the transceiver in an optical line terminal can be extended according to a bandwidth requirement in a stepwise manner, and time division multiplexing (TDM) and wavelength division multiplexing (WDM) are effectively combined has been reported (for example, see Non Patent Literature 1).
The wavelength tunable WDM/TDM-PON has recently attracted attention as a system in which a gradual total band increase and flexible load distribution are possible according to a user's demand as in Non Patent Literature 2, and at the time of the gradual total band increase, a dynamic wavelength and bandwidth allocation (DWBA) algorithm is used for update of an associated optical subscriber unit (OSU) by load distribution. The DWBA is implemented by a combination of uplink dynamic bandwidth allocation (DBA) from an optical network unit (ONU) in an associated OSU and wavelength switching for switching an associated OSU.
FIG. 1 is an example of a configuration diagram illustrating a wavelength tunable WDM/TDM-PON system related to the present disclosure. The wavelength tunable WDM/TDM-PON system related to the present disclosure includes an optical line terminal (OLT) 10 and an ONU 20. The OLT 10 includes a DWBA circuit 101, a multiplexing/separating unit 106, and an OSU 107. The OLT 10 is connected with the ONU 20 by a PON topology of a point-to-multipoint configuration using an optical multiplexer/demultiplexer 11 and an optical multiplexer/demultiplexer 12. Examples of the optical multiplexer/demultiplexer 11 and the optical multiplexer/demultiplexer 12 include a power splitter and a wavelength router. 13 indicates an optical fiber, 14 indicates an optical fiber, 15 indicates an optical fiber, 16 indicates an optical fiber, and 40 indicates a relay network.
The OLT 10 of FIG. 1 includes m line card OSUs 107 that transmit and receive m types of wavelength sets of λ1d and λ1u to λmd and λmu, the DWBA circuit 101, and the multiplexing/separating unit 106. The OSUs #1 to #m transmit and receive wavelength signals of λ1d and λ1u to λmd and λmu transmitted from the ONU 20. The OLT 10 is connected to h ONUs 20 through the optical multiplexer/demultiplexer 11, the optical multiplexer/demultiplexer 12, the optical fiber 13, the optical fiber 14, the optical fiber 15, and the optical fiber 16, and each of the ONUs 20 performs transmission and reception using any one of wavelength sets λ1d and λ1u to λmd and λmu each of which serves as a set of downlink and uplink wavelengths. Each of the ONUs 20 can switch wavelengths of λ1d and λ1u to λmd and λmu according to an instruction given from the OLT 10 and perform transmission and reception.
An uplink signal from a communication device installed in a house of a user is input to each of the ONUs 20, and transmitted as an uplink optical signal through an optical transceiver in the ONU 20. Since the uplink signals are multiplexed to one optical fiber 13 from the optical multiplexer/demultiplexer 11 at the ONU 20 side toward the OLT 10, the OLT 10 calculates and controls a transmission time and a transmission duration of the uplink signal transmitted by each of the ONUs 20 so that the uplink signals do not overlap. Uplink signal 1 to m received by the OSUs #1 to #m are aggregated and multiplexed into one uplink signal by the multiplexing/separating unit 106 in the OLT 10, and the uplink signal is transmitted to the relay network 40 side. On the other hand, a downlink signal to be transmitted from the relay network 40 side to the ONUs 20 is separated into downlink signals 1 to m to be transmitted to the OSUs #1 to #m through the multiplexing/separating unit 106 based on destination ONU 20 information described in the downlink signal and information of the OSU 107 to which the ONU 20 belongs. The separated downlink signals 1 to m are transmitted to the ONUs 20 at wavelengths of λ1d and λ1u to λmd and λmu of the OSUs #1 to #m. The downlink signals are broadcast to the ONUs 20 at the wavelengths of the OSUs 107, but since transmission and reception wavelengths of the ONU 20 are set to transmission and reception wavelengths of the associated OSUs 107, the ONU 20 selects information addressed to its own device from the received wavelength signal, and the ONU 20 outputs the selected information to the communication device in the house of the user.
The DWBA circuit 101 includes a DWBA calculating unit 103, a switching instruction signal generating unit 102, a control signal transmitting unit 104, and a request signal receiving unit 105. The request signal receiving unit 105 receives signals including a bandwidth request transmitted from the ONUs 20 through the OSUs 107, the DWBA calculating unit 103 calculates transmission times and transmission durations of uplink data signals and the request signals allocated to the ONUs 20 based on the request, and the switching instruction signal generating unit 102 generates an instruction signal storing the information, and causes the control signal transmitting unit 104 to transmit the instruction signal to each of the ONUs 20 through each of the OSUs 107. Further, the DWBA calculating unit 103 manages connection information of a PON zone, that is, a zone in which a plurality of ONUs 20 are connected with a plurality of OSUs 107 through the optical fiber 13, the optical fiber 14, the optical fiber 15, the optical fiber 16, the optical multiplexer/demultiplexer 11, and the optical multiplexer/demultiplexer 12. The downlink signal output from the multiplexing/separating unit 106 is relayed through the OSU 107 and transmitted to the ONU 20. When a wavelength used for communication by the ONU 20, the DWBA calculating unit 103 instructs the multiplexing/separating unit 106 to change the OSU 107 that relays the downlink signal to be transmitted to the ONU 20 that has changed the communication wavelength.
FIG. 2 illustrates a configuration of the ONU 20. The ONU 20 includes a data receiving unit 201, a data transmitting unit 208, an uplink buffer memory 202, a downlink buffer memory 209, a destination analysis selection receiving unit 210, a frame transmission control unit 203, a frame assembly transmitting unit 204, a wavelength tunable optical transceiver 205, a request bandwidth calculating unit 206, a request signal transmitting unit 207, an instruction signal receiving unit 211, and a wavelength switching control unit 212.
The uplink signal from the user is received by the data receiving unit 201 and temporarily stored in the uplink buffer memory 202. The frame transmission control unit 203 transfers the uplink signal to the frame assembly transmitting unit 204 according to the transmission time and the transmission duration of the uplink signal instructed by instruction signal given from the OLT 10. The frame assembly transmitting unit 204 configures a frame format necessary for transmitting the signal to the OLT 10 in a PON configuration, and transfers the frame format to the wavelength tunable optical transceiver 205. Here, the PON configuration refers to a configuration in which the OLT 10 and a plurality of ONUs 20 are provided, and the OLT 10 is optically connected with the ONUs 20 through the optical fiber and the optical multiplexer/demultiplexer. The wavelength tunable optical transceiver 205 performs conversion to an optical signal at any one of the wavelengths λ1d and λ1u to λmd and λmu instructed by the wavelength switching control unit 212, and transmits the optical signal to the OLT 10. The wavelength tunable optical transceiver 205 receives the downlink signal from the OSU 107 by selecting the instructed wavelength, and the destination analysis selection receiving unit 210 analyzes the destination of the downlink signal, selects only information addressed to its own device, and stores the selected information in the downlink buffer memory 209. The data transmitting unit 208 transmits the information stored in the downlink buffer memory 209 to the user as the downlink signal.
The wavelength tunable optical transceiver 205 receives the instruction signal from the OLT 10, converts the instruction signal into an electrical signal, and transfers the electrical signal to the instruction signal receiving unit 211. The instruction signal receiving unit 211 analyzes instruction content of the instruction signal, and transfers a switching destination wavelength and a switching execution instruction to the wavelength switching control unit 212 at an instruction time when a wavelength switching instruction, a wavelength after switching, and a switching start time are included in the instruction signal. The wavelength switching control unit 212 switches the wavelength of the wavelength tunable optical transceiver 205 according to wavelength switching control. The OLT 10 receives information about a bandwidth, requested by the ONU 20, from the ONU 20 and uses the information for bandwidth allocation. There are various methods, and, for example, an instruction may be given using the request bandwidth so that the request bandwidth information is transmitted to the OLT 10, and the ONU 20 may cause the request bandwidth information to be transmitted to the OLT 10 to be described in the request signal according to the instruction. In this case, upon receiving an instruction signal to request transmission of the request signal, the instruction signal receiving unit 211 instructs the request signal transmitting unit 207 to generate the request signal. The request signal transmitting unit 207 instructs the request bandwidth calculating unit 206 to calculate a bandwidth to be requested. The request bandwidth calculating unit 206 measures a data amount of the uplink signal stored in the buffer memory, decides a request bandwidth amount based on the data amount, and transfers the request bandwidth amount to the request signal transmitting unit 207. The request signal transmitting unit 207 generates a request signal in which the request bandwidth amount is described, and transfers the request signal to the frame transmission control unit 203.
The instruction signal may include both information about a transmission start time of a request signal and information about a transmission duration of the request signal. In this case, the instruction signal receiving unit 211 transfers both the information about the transmission start time of the request signal and the information about the transmission duration of the request signal included in the instruction signal to the frame transmission control unit 203, The frame transmission control unit 203 transfers the request signal to the frame assembly transmitting unit 204 at an instructed time, and the request signal is transmitted to the OLT 10 through the wavelength tunable optical transceiver 205. Further, the instruction signal transmitted from the OLT 10 includes a transmission start time and a transmission duration for transmitting the uplink signal received from the user side by the ONU 20 to the OLT 10. The instruction signal receiving unit 211 transfers the information about the transmission start time of the uplink signal and the information about the transmission duration of the uplink signal included in the instruction signal to the frame transmission control unit 203, and the frame transmission control unit 203 extracts the uplink signal from the buffer memory at the instructed time, and transfers the uplink signal to the frame assembly transmitting unit 204 during a period of time corresponding to the transmission duration, and the uplink signal is transmitted to the OLT 10 through the wavelength tunable optical transceiver 205.
In Non Patent Literature 3, redundancy of the OSU 107 is described as a function for increasing reliability and availability of the OLT 10. Two or more OSUs 107 are mounted in the OLT 10, and when an abnormality occurs in a certain OSU 107, communication is recovered by allocating uplink and downlink wavelengths of another OSU 107 as uplink and downlink wavelengths used by the ONU 20 to which uplink and downlink wavelengths used by the abnormal OSU 107 are allocated, and thus it is possible to prevent a communication interruption period of time when an abnormality occurs in the OSU 107.
However, in Non Patent Literature 3, the wavelength of the OSU 107 is fixed, and when an abnormality occurs in the OSU 107, communication interruption can be prevented by switching the wavelength of the ONU 20, but when the operation of the optical transceiver of the OSU 107 is stopped due to the occurrence of an abnormality in the OSU 107 or the downlink signal is lost due to a power supply interruption of the OSU 107 or the like, the ONU 20 itself needs to have an operation of selecting a switching destination OSU 107 and connecting itself to the switching destination OSU 107. In other words, the ONU 20 needs to be equipped with a mechanism for autonomously controlling the wavelength switching such that communication is recovered, but this increases the cost of the ONU 20 since the function related to the autonomous wavelength switching and the connection state recovery of the ONU 20 is mounted compared to the ONU 20 that constantly performs the wavelength switching according to the instruction given from the OLT 10. Thus, when an abnormality occurs, the mechanism for switching the wavelength has to have a simple configuration capable of suppressing the cost increase.
Further, when an abnormality occurs in the OSU 107, the ONU 20 allocated to the abnormal OSU 107 hardly receive the downlink wavelength signal of the abnormal OSU 107 and the switching instruction and hardly receives anything from the other OSUs 107 immediately. It is because the wavelength tunable optical transceiver 205 excludes the ONU 20 associated with the abnormal OSU 107, for example, using a wavelength filter in order to avoid interference with downlink wavelengths of the other OSUs 107 having different wavelengths. Thus, as described above, at a point in time at which an abnormality occurs in the OSU 107, and the ONU 20 allocated to the abnormal OSU 107 needs to recover communication through the wavelength switching, it is necessary to provide a device that selects the switching destination wavelength in a state in which information and an instruction related to the switching destination wavelength are not obtained.